Section (2) syslog
Name
syslog, klogctl — read and/or clear kernel message ring buffer; set console_loglevel
Synopsis
int
syslog( |
int type, |
char *bufp, | |
int len) ; |
/* No wrapper provided in glibc */ /* The glibc interface */ #include <sys/klog.h>
int
klogctl( |
int type, |
char *bufp, | |
int len) ; |
DESCRIPTION
![]() |
Note |
---|---|
Probably, you are looking for the C library
function |
This page describes the kernel syslog
() system call, which is used to
control the kernel printk
()
buffer; the glibc wrapper function for the system call is
called klogctl
().
The kernel log buffer
The kernel has a cyclic buffer of length LOG_BUF_LEN
in which messages given as
arguments to the kernel function printk
() are stored (regardless of their
log level). In early kernels, LOG_BUF_LEN
had the value 4096; from
kernel 1.3.54, it was 8192; from kernel 2.1.113, it was
16384; since kernel 2.4.23/2.6, the value is a kernel
configuration option (CONFIG_LOG_BUF_SHIFT
, default value
dependent on the architecture). Since Linux 2.6.6, the size
can be queried with command type 10 (see below).
Commands
The type
argument determines the action taken by this function. The
list below specifies the values for type
. The symbolic names are
defined in the kernel source, but are not exported to user
space; you will either need to use the numbers, or define
the names yourself.
SYSLOG_ACTION_CLOSE
(0)-
Close the log. Currently a NOP.
SYSLOG_ACTION_OPEN
(1)-
Open the log. Currently a NOP.
SYSLOG_ACTION_READ
(2)-
Read from the log. The call waits until the kernel log buffer is nonempty, and then reads at most
len
bytes into the buffer pointed to bybufp
. The call returns the number of bytes read. Bytes read from the log disappear from the log buffer: the information can be read only once. This is the function executed by the kernel when a user program reads/proc/kmsg
. SYSLOG_ACTION_READ_ALL
(3)-
Read all messages remaining in the ring buffer, placing them in the buffer pointed to by
bufp
. The call reads the lastlen
bytes from the log buffer (nondestructively), but will not read more than was written into the buffer since the last clear ring buffer command (see command 5 below)). The call returns the number of bytes read. SYSLOG_ACTION_READ_CLEAR
(4)-
Read and clear all messages remaining in the ring buffer. The call does precisely the same as for a
type
of 3, but also executes the clear ring buffer command. SYSLOG_ACTION_CLEAR
(5)-
The call executes just the clear ring buffer command. The
bufp
andlen
arguments are ignored.This command does not really clear the ring buffer. Rather, it sets a kernel bookkeeping variable that determines the results returned by commands 3 (
SYSLOG_ACTION_READ_ALL
) and 4 (SYSLOG_ACTION_READ_CLEAR
). This command has no effect on commands 2 (SYSLOG_ACTION_READ
) and 9 (SYSLOG_ACTION_SIZE_UNREAD
). SYSLOG_ACTION_CONSOLE_OFF
(6)-
The command saves the current value of
console_loglevel
and then setsconsole_loglevel
tominimum_console_loglevel
, so that no messages are printed to the console. Before Linux 2.6.32, the command simply setsconsole_loglevel
tominimum_console_loglevel
. See the discussion of/proc/sys/kernel/printk
, below.The
bufp
andlen
arguments are ignored. SYSLOG_ACTION_CONSOLE_ON
(7)-
If a previous
SYSLOG_ACTION_CONSOLE_OFF
command has been performed, this command restoresconsole_loglevel
to the value that was saved by that command. Before Linux 2.6.32, this command simply setsconsole_loglevel
todefault_console_loglevel
. See the discussion of/proc/sys/kernel/printk
, below.The
bufp
andlen
arguments are ignored. SYSLOG_ACTION_CONSOLE_LEVEL
(8)-
The call sets
console_loglevel
to the value given inlen
, which must be an integer between 1 and 8 (inclusive). The kernel silently enforces a minimum value ofminimum_console_loglevel
forlen
. See the log level section for details. Thebufp
argument is ignored. SYSLOG_ACTION_SIZE_UNREAD
(9) (since Linux 2.4.10)-
The call returns the number of bytes currently available to be read from the kernel log buffer via command 2 (
SYSLOG_ACTION_READ
). Thebufp
andlen
arguments are ignored. SYSLOG_ACTION_SIZE_BUFFER
(10) (since Linux 2.6.6)-
This command returns the total size of the kernel log buffer. The
bufp
andlen
arguments are ignored.
All commands except 3 and 10 require privilege. In Linux
kernels before 2.6.37, command types 3 and 10 are allowed
to unprivileged processes; since Linux 2.6.37, these
commands are allowed to unprivileged processes only if
/proc/sys/kernel/dmesg_restrict
has the
value 0. Before Linux 2.6.37, privileged means that the
caller has the CAP_SYS_ADMIN
capability. Since Linux 2.6.37, privileged means that the
caller has either the CAP_SYS_ADMIN
capability (now deprecated
for this purpose) or the (new) CAP_SYSLOG
capability.
/proc/sys/kernel/printk
/proc/sys/kernel/printk
is
a writable file containing four integer values that
influence kernel printk
()
behavior when printing or logging error messages. The four
values are:
console_loglevel
-
Only messages with a log level lower than this value will be printed to the console. The default value for this field is
DEFAULT_CONSOLE_LOGLEVEL
(7), but it is set to 4 if the kernel command line contains the word quiet, 10 if the kernel command line contains the word debug, and to 15 in case of a kernel fault (the 10 and 15 are just silly, and equivalent to 8). The value ofconsole_loglevel
can be set (to a value in the range 1–8) by asyslog
() call with atype
of 8. default_message_loglevel
-
This value will be used as the log level for
printk
() messages that do not have an explicit level. Up to and including Linux 2.6.38, the hard-coded default value for this field was 4 (KERN_WARNING
); since Linux 2.6.39, the default value is a defined by the kernel configuration optionCONFIG_DEFAULT_MESSAGE_LOGLEVEL
, which defaults to 4. minimum_console_loglevel
-
The value in this field is the minimum value to which
console_loglevel
can be set. default_console_loglevel
-
This is the default value for
console_loglevel
.
The log level
Every printk
() message has
its own log level. If the log level is not explicitly
specified as part of the message, it defaults to default_message_loglevel
.
The conventional meaning of the log level is as
follows:
Kernel constant | Level value | Meaning |
KERN_EMERG |
0 | System is unusable |
KERN_ALERT |
1 | Action must be taken immediately |
KERN_CRIT |
2 | Critical conditions |
KERN_ERR |
3 | Error conditions |
KERN_WARNING |
4 | Warning conditions |
KERN_NOTICE |
5 | Normal but significant condition |
KERN_INFO |
6 | Informational |
KERN_DEBUG |
7 | Debug-level messages |
The kernel printk
()
routine will print a message on the console only if it has
a log level less than the value of console_loglevel
.
RETURN VALUE
For type
equal to
2, 3, or 4, a successful call to syslog
() returns the number of bytes read.
For type
9,
syslog
() returns the number of
bytes currently available to be read on the kernel log
buffer. For type
10,
syslog
() returns the total size
of the kernel log buffer. For other values of type
, 0 is returned on
success.
In case of error, −1 is returned, and errno
is set to indicate the error.
ERRORS
- EINVAL
-
Bad arguments (e.g., bad
type
; or fortype
2, 3, or 4,buf
is NULL, orlen
is less than zero; or fortype
8, thelevel
is outside the range 1 to 8). - ENOSYS
-
This
syslog
() system call is not available, because the kernel was compiled with theCONFIG_PRINTK
kernel-configuration option disabled. - EPERM
-
An attempt was made to change
console_loglevel
or clear the kernel message ring buffer by a process without sufficient privilege (more precisely: without theCAP_SYS_ADMIN
orCAP_SYSLOG
capability). ERESTARTSYS
-
System call was interrupted by a signal; nothing was read. (This can be seen only during a trace.)
CONFORMING TO
This system call is Linux-specific and should not be used in programs intended to be portable.
NOTES
From the very start, people noted that it is unfortunate that a system call and a library routine of the same name are entirely different animals.
COLOPHON
This page is part of release 4.16 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
t Copyright (C) 1995 Andries Brouwer (aebcwi.nl) and Copyright (C) 2012, 2014 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Written 11 June 1995 by Andries Brouwer <aebcwi.nl> 2008-02-15, Jeremy Kerr <jkozlabs.org> Add info on command type 10; add details on types 6, 7, 8, & 9. 2008-02-15, Michael Kerrisk <mtk.manpagesgmail.com> Update LOG_BUF_LEN details; update RETURN VALUE section. |
Section (3) syslog
Name
closelog, openlog, syslog, vsyslog — send messages to the system logger
Synopsis
#include <syslog.h>
void
openlog( |
const char *ident, |
int option, | |
int facility) ; |
void
syslog( |
int priority, |
const char *format, | |
...) ; |
void
closelog( |
void) ; |
void
vsyslog( |
int priority, |
const char *format, | |
va_list ap) ; |
![]() |
Note | ||||
---|---|---|---|---|---|
|
DESCRIPTION
openlog()
openlog
() opens a
connection to the system logger for a program.
The string pointed to by ident
is prepended to every
message, and is typically set to the program name. If
ident
is NULL, the
program name is used. (POSIX.1-2008 does not specify the
behavior when ident
is NULL.)
The option
argument specifies flags which control the operation of
openlog
() and subsequent
calls to syslog
(). The
facility
argument
establishes a default to be used if none is specified in
subsequent calls to syslog
().
The values that may be specified for option
and facility
are described
below.
The use of openlog
() is
optional; it will automatically be called by syslog
() if necessary, in which case
ident
will default
to NULL.
syslog() and vsyslog()
syslog
() generates a log
message, which will be distributed by syslogd(8).
The priority
argument is formed by ORing together a facility
value and a
level
value
(described below). If no facility
value is ORed into
priority
, then the
default value set by openlog
() is used, or, if there was no
preceding openlog
() call, a
default of LOG_USER
is
employed.
The remaining arguments are a format
, as in printf(3), and any
arguments required by the format
, except that the
two-character sequence %m
will be replaced by the
error message string strerror
(errno
). The format string need not include
a terminating newline character.
The function vsyslog
()
performs the same task as syslog
() with the difference that it
takes a set of arguments which have been obtained using the
stdarg(3) variable
argument list macros.
closelog()
closelog
() closes the file
descriptor being used to write to the system logger. The
use of closelog
() is
optional.
Values for option
The option
argument to openlog
() is a
bit mask constructed by ORing together any of the following
values:
LOG_CONS
-
Write directly to the system console if there is an error while sending to the system logger.
LOG_NDELAY
-
Open the connection immediately (normally, the connection is opened when the first message is logged). This may be useful, for example, if a subsequent chroot(2) would make the pathname used internally by the logging facility unreachable.
LOG_NOWAIT
-
Don_zsingle_quotesz_t wait for child processes that may have been created while logging the message. (The GNU C library does not create a child process, so this option has no effect on Linux.)
LOG_ODELAY
-
The converse of
LOG_NDELAY
; opening of the connection is delayed untilsyslog
() is called. (This is the default, and need not be specified.) LOG_PERROR
-
(Not in POSIX.1-2001 or POSIX.1-2008.) Also log the message to
stderr
. LOG_PID
-
Include the caller_zsingle_quotesz_s PID with each message.
Values for facility
The facility
argument is used to specify what type of program is logging
the message. This lets the configuration file specify that
messages from different facilities will be handled
differently.
LOG_AUTH
-
security/authorization messages
LOG_AUTHPRIV
-
security/authorization messages (private)
LOG_CRON
-
clock daemon (cron and at)
LOG_DAEMON
-
system daemons without separate facility value
LOG_FTP
-
ftp daemon
LOG_KERN
-
kernel messages (these can_zsingle_quotesz_t be generated from user processes)
LOG_LOCAL0
throughLOG_LOCAL7
-
reserved for local use
LOG_LPR
-
line printer subsystem
LOG_MAIL
-
mail subsystem
LOG_NEWS
-
USENET news subsystem
LOG_SYSLOG
-
messages generated internally by syslogd(8)
LOG_USER
(default)-
generic user-level messages
LOG_UUCP
-
UUCP subsystem
Values for level
This determines the importance of the message. The levels are, in order of decreasing importance:
LOG_EMERG
-
system is unusable
LOG_ALERT
-
action must be taken immediately
LOG_CRIT
-
critical conditions
LOG_ERR
-
error conditions
LOG_WARNING
-
warning conditions
LOG_NOTICE
-
normal, but significant, condition
LOG_INFO
-
informational message
LOG_DEBUG
-
debug-level message
The function setlogmask(3) can be used to restrict logging to specified levels only.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
openlog (), closelog () |
Thread safety | MT-Safe |
syslog (), vsyslog () |
Thread safety | MT-Safe env locale |
CONFORMING TO
The functions openlog
(),
closelog
(), and syslog
() (but not vsyslog
()) are specified in SUSv2,
POSIX.1-2001, and POSIX.1-2008.
POSIX.1-2001 specifies only the LOG_USER
and LOG_LOCAL*
values for facility
. However, with the
exception of LOG_AUTHPRIV
and
LOG_FTP
, the other facility
values appear on most
UNIX systems.
The LOG_PERROR
value for
option
is not
specified by POSIX.1-2001 or POSIX.1-2008, but is available
in most versions of UNIX.
NOTES
The argument ident
in the call of openlog
() is
probably stored as-is. Thus, if the string it points to is
changed, syslog
() may start
prepending the changed string, and if the string it points to
ceases to exist, the results are undefined. Most portable is
to use a string constant.
Never pass a string with user-supplied data as a format, use the following instead:
syslog(priority, %s, string);
COLOPHON
This page is part of release 4.16 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Written Feb 1994 by Steve Greenland (stevegrneosoft.com) and Copyright 2001, 2017 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Updated 1999.12.19 by Karl M. Hegbloom <karlhegdebian.org> Updated 13 Oct 2001, Michael Kerrisk <mtk.manpagesgmail.com> Added description of vsyslog Added descriptions of LOG_ODELAY and LOG_NOWAIT Added brief description of facility and option arguments Added CONFORMING TO section 2001-10-13, aeb, minor changes Modified 13 Dec 2001, Martin Schulze <joeyinfodrom.org> Modified 3 Jan 2002, Michael Kerrisk <mtk.manpagesgmail.com> |